1. Related Applications
The present application claims priority to Japanese Patent Application Number 2007-313620, filed Dec. 4, 2007, the entirety of which is hereby incorporated by reference.
2. Field of the Invention
The present invention relates to distortion-corrected image generation units and distortion-corrected image generation methods, and in particular, relates to a distortion-corrected image generation unit and a distortion-corrected image generation method suitable for correcting the distortion of an image captured by a fish-eye lens.
3. Description of the Related Art
In general, distortion generated in an image captured using a lens, for example, a fish-eye lens (hereinafter called a distorted image), the focal length of which is short, is corrected using the internal parameters of the used lens. FIG. 23 is a schematic diagram showing known distortion correction. In Part (a) of FIG. 23, reference letters XYZ denote a camera coordinate system (an origin O, an optical axis direction Z, and two directions X and Y on a plane that includes a surface of a lens). A reference letter P denotes the coordinates of an actual object in the camera coordinate system XYZ. Reference letter Θ denotes an incident angle with respect to the optical axis direction Z in the direction toward the object. A reference letter o denotes an image principal point that is an internal parameter of a camera. A reference letter p denotes a pixel position in a distorted image. A reference letter q denotes a pixel position, corresponding to the pixel position p, in an image obtained by correcting the distortion of the distorted image (hereinafter called a distortion-corrected image). A reference letter r denotes an image height that represents the distance between the pixel position q and the image principal point o. In this case, the lens surface includes the center of the lens and is perpendicular to the optical axis direction Z.
Internal parameters used in the known distortion correction include, for example, the coordinates of the image principal point o and distortion correction parameters k1, k2, k3, k4, and k5 shown in Part (a) of FIG. 23. The distortion correction parameters k1, k2, k3, k4, and k5 are the coefficients of individual orders of an equation of higher degree for defining the image height r in the distortion-corrected image, where the incident angle Θ is a variable, as shown in Part (b) of FIG. 23. According to the equation of higher degree in Part (b) of FIG. 23, the pixel position p in a distorted image captured at the same incident angle Θ is corrected to the pixel position q with the same image height r in a distortion-corrected image regardless of the direction from the image principal point o. In other words, the equation of higher degree in Part (b) of FIG. 23 is based on an idea that the degree of distortion generated in a distorted image depends only on the incident angle Θ. In this case, for example, Japanese Patent No. 3286306 discloses an image generation unit that includes a calibration unit in which a lens distortion correction value of a camera is used as a parameter.
However, in practice, the degree of distortion generated in a distorted image does not depend only on the incident angle Θ. Thus, a problem exists in that the distortion of a distorted image cannot be accurately corrected using the equation of higher degree in Part (b) of FIG. 23. When the lens surface is exactly parallel to a surface of an image pickup device of a charge coupled device (CCD) (respective distances between all positions on the image pickup device surface and the lens surface are the same as a predetermined distance (a predetermined gap width)), the degree of distortion generated in a distorted image depends only on the incident angle Θ. However, in practice, the lens surface is not exactly parallel to the image pickup device surface due to manufacturing tolerances, thereby resulting in such a problem.
When the lens surface is parallel to the image pickup device surface, respective distances between all positions on the image pickup device surface and the lens surface are the same as the predetermined distance, as described above. However, when the lens surface is not parallel to the image pickup device surface, errors with respect to the predetermined distance occur at all positions, except a certain position or any position on a certain straight line, on the image pickup device surface. Moreover, the error magnitude varies with the position on the image pickup device surface and is maximized at the edge of the image pickup device surface. Moreover, the larger the error magnitude, the lower the accuracy of distortion correction. Thus, a significant problem is that distortion cannot be accurately corrected at the edge of a distorted image in which distortion is likely to occur and needs to be accurately corrected.
In view of the aforementioned problems, it is an object of the present invention to accurately correct the distortion of a distorted image even when a surface of a lens is not exactly parallel to a surface of an image pickup device due to manufacturing tolerances.